1. Field of the Invention
This invention relates to a method for forming an insulating coating on surfaces of copper materials employed in various forms such as wires, rods, stranded cables, bands, tubes and pipes. More specifically, this invention provides a method for forming a tough, heat-resistant, electrical insulating layer on a surface of a copper material by anodizing the copper material in an acid bath of a hexacyanoiron complex.
2. Description of the Related Art
A variety of methods has heretofore been proposed for the formation of an electrical insulating coating layer (hereinafter simply called "electrical insulating layer") on surfaces of various materials, including the following methods:
i) Coating of an organic material:
For example, Scotch.RTM. tapes (product of 3M Co., St. Paul, Minn., U.S.A.) are made of a polyester, PTFE or polyimide material and use a thermosetting silicone rubber or an acrylic adhesive. Although they have an excellent withstand voltage (dielectric strength), their heat resistance is below 200.degree. C.
ii) Coating of an inorganic material: Proposed coatings include, for example, flexible coatings formed by firing glass fibers in combination with an organic substance rather than simply applying glass fibers; and coatings obtained by applying inorganic polymers which contain boron, silicon and/or oxygen and can be converted to ceramics when fired. These coatings are however thick and costly so that their use for electronic devices and equipment reduced in dimensions and improved in precision is unsuitable.
Incidentally, as a simple and easy method for forming a reliable, electrical insulating layer, there is a method in which 0.1-mm thick mica is coated with an adhesive and inorganic powder. This method however involves problems, for example, in coil winding or the like because the coating thus formed has poor adhesion to the substrate. A limitation is therefore imposed on its practical utility.
iii) Different from the above-described coating of an organic material or an inorganic material, there are methods for directly forming an electrical insulating layer on a surface of a conductor.
These methods include, for example, formation of alumite (i.e., anodic oxidation coating of aluminum) and anodization. These methods are both applicable only to those made of an aluminum-based material. When the degree of wire drawing becomes 0.5 mm or smaller in diameter, extreme difficulties are encountered and an increase in product cost is unavoidable. These methods therefore have poor practical utility.
Other methods have also been proposed, in which a copper material having excellent conductivity and excellent workability such as wire drawability is made electrically insulating at a surface thereof by chemical conversion or anodization. These methods however also have problems to be described below, so that their use in actual production is inhibited.
In chemical conversion, an electrolytic bath is prepared generally by adding a single alkali salt at a high concentration and an oxidizing agent, and a copper material to be treated is dipped at a high temperature in the electrolytic bath so that a layer of cupric oxide (CuO) is formed on a surface of the copper material. This method however requires not only a long time for the chemical conversion but also a rather high cost for the reagents, and its productivity is therefore poor.
In anodization, an electrical insulating layer composed of cupric oxide (CuO) is formed on a surface of a copper material at a high current density in a alkaline solution of a high concentration in order to ensure high productivity. In this anodization, cupric oxide thus formed is instantaneously redissolved even by a slightest variation in conditions (alkali concentration, current density), whereby its process control is extremely difficult. Anodization is generally conducted by setting the alkali concentration of the alkaline bath at a high level and maintaining the current density also at a high level.
Another serious problem of the above-mentioned anodization resides in that an anodized product must be washed thoroughly with water. If an alkali component should remain on the product, the alkali component may cause an insulation failure due to its hygroscopic action. The anodization mentioned above is therefore considered to have poor practical utility when large facilities, a lots of water and waste water treatment, all of which are required for the through washing with water, are taken into consideration. This water washing poses an especially serious problem when the product has a shape inconvenient for washing as in the case of a stranded cable, unavoidably resulting in extremely low productivity.
With a view toward overcoming the above-described drawbacks in the anodization of copper materials, there has been proposed an anodization method for a copper material in which plural alkaline bathes are arranged in a linear pattern, the alkali concentrations of the individual bathes are successively lowered in the travelling direction of the copper material, and the average anode current in each bath is lowered (Japanese Patent Application Laid-Open No. 31099/1983). In the conventional anodization methods including the improved anodization methods described right above, an electrical insulating layer formed on a surface of a copper material and composed of cupric oxide (CuO) has a large thickness and is weak against external strains so that it tends to develop cracks. Moreover, the heat resistance of the electrical insulating layer and its adhesion strength to the substrate are insufficient. For these reasons, the conventional anodization methods for copper materials cannot meet, for example, the stringent requirements for coils and the like that an extremely thin, heat-resistant, peel-free, electrical insulating layer must be formed.